Bug 821078

Description of problem:

For Katello, we plan to support system (consumer) groups.  As part of that feature, we'd like to be able to go to a group (in katello), observe the list of errata that are applicable to the consumers in that group, select 1 or more of the errata and initiate an install.

In order to do this efficiently, we'd like to get a couple of additional APIs in to pulp, including:

1. Given a list of consumer ids, retrieve the list of errata that are applicable (i.e. that need to be installed) on 1 or more of the consumers in the list.  For each errata returned, include the list of consumer IDs that the errata applies to.

2. Given a consumer group id, retrieve the list of errata that are applicable (i.e. that need to be installed) on 1 or more of the consumers in the group.  For each errata returned, include the list of consumer IDs that the errata applies to.

2 is a simple variation of 1 and I'd view it as the more important of the APIs, since Katello is using Pulp consumer groups.